This disclosure relates generally to gas turbines and, more specifically, to flexible chordal hinge seals for sealing turbine nozzles within a gas turbine.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles include an annular array or assemblage of cast nozzle segments, each including one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer band portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array between annular inner and outer bands. An outer shroud or retaining ring coupled to the outer band of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular inner support ring is engaged by the inner band and supports the first-stage nozzles against axial movement.
In an exemplary arrangement, forty-eight cast nozzle segments are provided with one vane per segment. The annular array of segments are sealed one to the other along adjoining circumferential edges by side seals. The side seals form a seal between high and low pressure regions by extending radially inwardly of the inner band and radially outwardly of the outer band. The high pressure region is found in the compressor discharge air, and the low pressure region is found in the hot gases of combustion of the hot gas flow path.
The nozzle segments also include inner and outer chordal hinge seals. The inner chordal hinge seals are used to seal between the inner band of the first-stage nozzles and an axially facing surface of the inner support ring. Each inner chordal hinge seal includes an inner rail extending radially inwardly from the inner band portion and a projection extending along the inner rail that runs linearly along a chord line of the inner band portion of each nozzle segment. This projection lies in sealing engagement with the axially opposite facing sealing surface of the inner support ring. The inner chordal hinge seals also act as hinges to allow the first-stage nozzles to move forward and aft as the inner support ring and the compressor discharge case undergo thermal expansion.
In addition, the outer sidewall chordal hinge seals are used to seal between the outer band of the first-stage nozzles and an axially facing surface of the outer shroud. Each outer chordal hinge seal includes an outer rail extending radially outwardly from the outer band portion and a projection extending along the outer rail that runs linearly along a chord line of the outer band portion of each nozzle segment. This projection lies in sealing engagement with the axially opposite facing sealing surface of the outer shroud. The outer chordal hinge seals also act as hinges to allow the first-stage nozzles to move forward and aft as the outer support ring or shroud and the compressor discharge case undergo thermal expansion.
During operation and/or repair of the first-stage nozzle, it has been found that both the outer and inner chordal hinge seals tend to experience warpage due to temperature differences across their rails. In particular, the seals tend to bow aft in the center and bow forward on the intersegment ends of the rails. Such warpage can cause gaps to form between the inner and outer chordal hinge seals and the respective sealing surfaces of the inner support ring and the outer shroud. These gaps can enable leakage of the compressor discharge cooling air into the hot gas flow path. This leakage can lead to problems such as increased production of NOx pollutants, hot gas ingestion past the chordal seals, and higher flowpath aero losses, which result in a lower heat rate.
Currently, supplemental seals are employed at the interface of the first-stage nozzles and the inner support ring/outer shroud to reduce the leakage flow past the chordal hinge seals. However, the use of such supplemental seals significantly adds to the complexity and cost of manufacturing gas turbines. A need therefore exists to develop a way of minimizing the leakage of fluid past the inner and outer sidewall chordal hinge seals without significantly increasing the cost and complexity of manufacturing gas turbines including such seals.